You need to remove the entire light cluster from the car.
There's a fastner on the inside of the boot, behind a flap in the boot liner.
Disconnect the wiring plug.
Using a combination of pulling from the outside and pushing the bolt from the inside, the entire cluster moves backwards and off the car.
The bulbs are mounted to a red plastic holder in the middle of the cluster which you can remove (it's sealed with a rubber seal but moisture will always get in!).